On this day in 2005, the Supreme Court decided Roper v. Simmons, one of its most important rulings on the issue of capital punishment. In Roper, the Court held that Eighth Amendment’s prohibition on “cruel and unusual punishments” forbids imposing the death penalty for a crime committed by someone under the age of 18.

In 1994, a Missouri court found Christopher Simmons guilty of a murder; he was seventeen at the time of the crime. After a jury sentenced him to death, Simmons pursued a series of appeals. In 2002, the Missouri Supreme Court stayed Simmons’ execution pending the outcome at the U.S. Supreme Court of Atkins v. Virginia, a case considering the constitutionality of executing offenders determined to be mentally retarded. The Supreme Court would hold in Atkins that executing the mentally retarded violated the 8th Amendment, in part because of their “diminished capacities to understand and process information, to communicate, to abstract from mistakes and learn from experience, to engage in logical reasoning, to control impulses, and to understand the reactions of others.” The Missouri Supreme Court pointed to this reasoning from Atkins, along with the recent state-level trend away from executing juveniles, as justification for reducing Simmons sentence to life without parole.

Missouri appealed the reduction in sentence to the U.S. Supreme Court, leading to the historic Roper decision striking down capital punishment for minors.

Writing for the a five-justice majority, Justice Kennedy drew on the same principles that had guided the Missouri Supreme Court. He referenced the growing number of states that were rejecting capital punishment for juvenile offenders as a factor in the Court’s reading of the Eighth Amendment. He also drew a parallel between the reasoning in Atkins and the case at hand, noting, “The differences between juvenile and adult offenders are too marked and well understood to risk allowing a youthful person to receive the death penalty despite insufficient culpability.”

Particularly controversial was Kennedy’s reference to the fact that the United States was an outlier in the international community on this issue. Most other countries had rejected the practice of executing juvenile offenders. Defending himself against accusations by Justice Scalia and others that references to foreign legal practices had no place in constitutional interpretation, Kennedy argued that “recognition of the aberrant place of United States on the global scene does not lessen our fidelity to the Constitution or our pride in its origins.”